Species invasion has a larger impact on stickleback reproduction than warming of breeding habitat

The invasion of foreign species into ecosystems is a growing human-induced problem. Global warming is expected to magnify the problem by facilitating invasions and amplifying the ecological impact of invaders. We investigated if rising water temperature influences the impact of an invading shrimp Palaemon elegans on the reproductive success of a native fish, the threespine stickleback Gasterosteus aculeatus. Using a 2 × 2 factorial design, we found the shrimp to reduce the probability that stickleback males built a nest, defended it against a perceived intruder, courted females and gained matings. This reduced the number of offspring they produced, independently of temperature. Males lost more weight in the presence of the shrimp, which indicates that the shrimp can reduce the number of breeding cycles that males can complete. Thus, the shrimp has both direct negative effects and the potential for indirect delayed effects. The impact of the shrimp on stickleback reproduction overrode that of warming, most likely because the stickleback has adapted to temperature fluctuations in shallow coastal waters, but not to a high abundance of the non-native shrimp. Interestingly, the shrimp did not reduce egg hatching success, which indicates that the stickleback overestimates the risk posed by the invader. Refraining from reproduction in the presence of shrimp may have been an adaptive strategy in the past, but not in the current environment where shrimp-free habitats may no longer be found. Thus, the past reaction norm – to avoid nesting in the presence of shrimp – may have become an ‘evolutionary trap’. These results emphasise the immediate threat that invading species can pose, which can be larger than the impact of warming, and stresses the importance of controlling their invasions.